20 MR J. Y. BUCHANAN ON THE 



§ 4. The Hydrometer in the ^^ Challenger'' Expedition. — The dispatch of the 

 Challenger Expedition was decided before the end of the year 1871, and Sir Wyville 

 Thomson, who was then Professor of Natural History in the University of Edinburgh, 

 was chosen for its leader. He did me a great honour and a very substantial service in 

 selecting me for the post of chemist and physicist of the expedition quite a year before 

 the date fixed for its departure. I cannot adequately express the gratitude which 

 I feel for the confidence which he thus showed in me, and for the privilege which 

 it gave me of taking an active part in this memorable expedition. The expedition 

 lasted less than four years, yet these years are fuller of recollections than all the rest 

 of my life. 



During the year which elapsed between my selection and my official appointment, 

 I occupied myself almost exclusively in preparing for my work at sea, and I considered 

 that the specific gravity of the water of the ocean, and its variations, would be one of the 

 most important matters for continuous observation. Here I had in view the variations 

 of specific gravity which occur in the open ocean and far from all influence of the land. 

 These were only imperfectly known, but there was reason to conclude that they were 

 confined within narrow limits. 



I chose the hydrometer, or " araeometer," as it is called abroad, because it appeared 

 to me to be the only type of instrument which furnished directly the information 

 demanded, namely, the specific gravity of the water, and that with the exactness 

 required when the variations of specific gravity are so small. 



Even at that early date indirect methods of all kinds were recommended to 

 me. In theory, any physical constant of a saline solution, the expression of 

 which includes a term depending on its specific gravity, can be used for this 

 purpose. But indirect methods «re, m the nature of things, affected with at 

 least a double quantity of eirors. There are the errors ivith which the datum 

 directly supplied by the vicarious method used is affected, and there are those which 

 affect the operation of comjja^'ison by which that datum obtains its densimetric 

 interpretation. 



I had then, and I have still, an instinctive dislike of all indirect methods in science ; 

 I therefore adhered to my own purpose, believing that, if nothing but manipulative 

 difficulties stood in the way, they could be overcome by perseverance and a determina- 

 tion not to accept defeat too readily ; and, as is so often the case, the difficulties 

 apprehended turned out to be in no way formidable. 



§ 5. In designing the hydrometer I decided that, in the values of the specific gravity 

 obtained with it, units in the fourth place of decimals must be exact, and that the 

 exactness should be pushed as far as possible into the fifth place. As a knowledge of 

 the physical constants of the instrument is of the first importance, I rejected the plan 

 of having a series of liydrometers, each to be used in the waters the specific gravity of 

 which corresponded to the limits of its scale. I deeided to have one hydrometer, made 

 of glass, in which the dimensions of the stem and of the body should be in such pro- 



